# Inputs

Inputs are the symbols you write on the left-hand-side of a rule diagram to check an atom's surroundings.\
In the SandPond engine, there are some in-built inputs that you can use straight away:

* `@` The centre of the diagram.
* `_` An empty space.
* `#` A non-empty space.
* `.` Any space.
* `*` Not a space (ie: the edge of the universe).
* `$` This element.

You can also [make your own input symbols](https://todepond.gitbook.io/spacetode/symbols) by using the following keywords:

* `symbol`
* `given`
* `select`
* `check`